Birth by Sleep Terra and Aqua's relationship *INCLUDES SPOILERS*

Their relationship is simply one of friendly classmates. There's nothing to suggest anything more.
If anything, Ven and Aqua have a deeper relationship than Terra and Aqua, one of a lost boy and something resembling a surrogate mother.

 

All this "relations" stuff came out from all the shipping that the fans made (and some of them have pretty nasty ideas when they do comics, and doujins *shivers)

^
This.

 

Roxas X Namine ... basically another Sora X Namine, even Roxas X Xion is basically Sora X Kairi (Xion is just how Sora remembers Kairi).

 

Sora x Kairi's the only one that's remotely canon, Roxas x Namine is more of implied due to them being the nobodies of a canon pairing. Aqua x Terra is more like a pairing made by the fans.

 

I am saying that Roxas is sora and Namine and Xion are Kairi, no matter what it is the same thing